The art of making and developing new uses for thermo optic devices continues to emerge. Presently, thermo optic devices are used as filters, switches, multiplexers, waveguides, and a host of other semiconductor and optical transmission devices.
With reference to FIGS. 1A and 1B, a prior art thermo optic device in the form of an optical waveguide is shown generally as 110. It comprises a grating 112 formed of a lower cladding 114, an upper cladding 116, an input waveguide 118, an output waveguide 120 and a grating waveguide and an optional resonator 122. As is known, the waveguides and resonator are formed of a material having a higher refractive index than that of the upper and lower claddings to propagate light therein during use. The grating 112 is disposed on a substrate 124. In many thermo optic devices the substrate is a printed circuit board or some form of silicon.
In forming the device, the lower cladding is deposited on the substrate. An intermediate layer, for the waveguides and resonator, is deposited on the lower cladding, photo patterned and etched. The upper cladding is deposited on the waveguides and resonator. In an alternate formation process, the lower cladding 206 is an oxidation of a silicon substrate with the waveguides, resonator and upper cladding being formed in the same manner.
The inherent characteristics of waveguides and resonators, such as their sizes, shapes, compositions, etc., may vary greatly from application to application. The characteristics of all waveguides and resonators, however, are generally selected in such a manner to eliminate crosstalk between the input and output waveguides at undesirable frequencies and to resonate signals (i.e., prolong and/or intensify) which allows transfer between the waveguides at desirable frequencies. The undesirable frequencies are not transferred between the two waveguides. The range of frequencies that are not transferred is determined by the properties of the grating, and is typically referred to as the bandwidth. The frequencies that are transferred are determined by the specific designs of the grating resonator waveguides.
In the representative prior art embodiment shown in FIG. 1B, the resonator 122 has a generally symmetrical tooth-shaped pattern. To set the center frequency, the grating corrugation period is adjusted by adjusting the pitch (distance) between the teeth.
As part of the task of setting the bandwidth, an aspect ratio is adjusted in an area where the waveguide and resonator front or face one another. It is not possible to change the bandwidth by only changing the aspect ratio. The grating strength changes the bandwidth and it is necessary to change the aspect ratio to allow the device to operate appropriately.
For example, in FIG. 1A, resonator 122 has a surface 123 facing a surface 119 of input waveguide 118. The aspect ratio (a.r.) in this area is defined as the area of the input waveguide surface to the area of the resonator surface (a.r.=area of input waveguide surface/area of resonator surface). A large bandwidth corresponds to a small aspect ratio while a small bandwidth corresponds to a large aspect ratio. Correspondingly, a large bandwidth can be achieved by either increasing the area of the resonator surface, decreasing the area of the input waveguide surface, or adjusting both surface areas in such a manner to achieve a relatively small ratio number. A small bandwidth can be achieved by either decreasing the area of the resonator surface, increasing the area of the input waveguide surface, or adjusting both surface areas in such a manner to achieve a relatively large ratio number. Even further, increases or decreases of surface area can be achieved by adjusting one or both of the surface dimensions of the waveguide or resonator surfaces. For example, depth “D” of surface 119 or 123 may be increased or decreased according to desired bandwidth.
In other words, to set the bandwidth, the strength of the grating between the input and output waveguides is increased. As the grating strength is increased, the difference in effective index for waveguides with and without gratings becomes increasingly difficult to maintain. The difference in effective index for coupled devices such as these is typically referred to as asynchronicity. The term asynchronicity indicates that the propagation constant at the resonant wavelength is different for the waveguide and grating, which limits the amount of light that can be coupled between them. The problem of asynchronicity becomes even more problematic when it is desirable to achieve polarization independent devices, as is required for commercial fiber optic components. In this case, coupling between the grating and waveguide requires synchronicity for both of the orthogonal polarization states of the system.
Methods for trimming the effective index of the waveguide to match the grating, or grating to match the waveguide, are required to achieve optimal performance from coupled systems such as the waveguide/grating coupler system. Trimming approaches have been defined elsewhere (See “Integrated-Optic Grating-Based Filters For Optical Communication Systems” by Jay Northrop Damask, Massachusetts Institute of Technology thesis, available Jul. 16, 1996, chapter 4), but are not generalized for addressing arbitrary waveguide combinations, or are not compatible with standard processing techniques.
Since the resonator 122 and the input and output waveguides 118, 120 are formed together during the same process steps as described above, the depth, D, of the resonator is essentially fixed as the same depth of the waveguides and therefore the asynchronicity limits the bandwidths and grating strengths that can be used.
Accordingly, the thermo optic arts desire waveguides having increased bandwidths that are relatively cheap and quick to produce without sacrifices in quality, reliability or longevity.